Such coatings are described in EP 1 096 040 A2, EP 905 279 A1, EP 995 816 636 A1 and EP 933 446 B1. In the last-named document in particular it is stated that the coating can be mixed with additives in the form of particles which guarantees the function of corrosion inhibitors, dry lubricants, color pigments or a combination of these functions.
When the coatings are produced these particles are subjected to a dispersion which forms a coating substance. This coating substance is applied to the substrate to be coated which is subsequently subjected to heat treatment, in order to achieve hardening of the coating. In this case a chemical conversion of the elements contained in the dispersion occurs simultaneously, which contributes to the formation of the ceramic framework of the coating.
Especially for the color pigments which are employed it has been shown that these are subject to a chemical change during the heat hardening or during subsequent thermal stress on the component, which also leads to a change in the optical appearance of the surface. This takes the form of the color of the surface changing during ongoing operation of the component. Such observations can typically be made during operation of thermally highly-stressed turbine blades or compressor blades of a gas turbine.